Acid rock drainage (ARD) is a natural process that occurs when sulfur-containing compounds in rock are exposed to air and water. When this process occurs in the context of mining operations, where sulfur-containing rocks are exposed as a result of open pit or underground mining, the process is referred to as acid mine drainage (AMD). The process produces acidic waters as a result of the oxidation of the minerals pyrite (FeS2) and pyrotite (FeS) and other sulfur-containing compounds, generating sulfuric acid. The oxidation is increased by the bioactivity of oxidizing bacteria, in particular Thiobacillus and Ferrobacillus. The pH of the acidic waters is typically about 2.1 to 3.5. This low pH causes the water to leach metals from the rock and soils in contact therewith. Other mine wastewaters resulting from the operation of a mine, whether an underground or an open pit mine, including the water used in the operating process of the mine and from mill clean-up, are also often highly acidic. All such wastewaters, including ARD and AMD, are collectively referred to in this specification as “acidic mine wastewaters.”
The contamination of water supplies by the metals in acidic mine wastewaters is a serious environmental concern. For example, the metals dissolved in such waters may kill fish and other aquatic life, and may pose human health hazards when they find their way into drinking water supplies.
Various processes have been proposed for reducing the metal content of wastewater. It is known that flocculating agents are of use in such processes. For example, U.S. Pat. No. 5,720,886 (Iwinski) discloses a process for removing metals from mine wastewaters that uses an anionic polymer and a flocculent.
One known method for treating acid mine drainage is the high density sludge (HDS) process. In this process, an excess amount of lime is applied to neutralize the acidity of the water and raise the pH to about 9–10. An appropriate flocculent is then added and the mixture is transferred to a clarifier, from which clean effluent is decanted from a sludge. The HDS process uses large amounts of lime, is very time-consuming, and requires the handling of a large quantity of sludge and further treatment of the sludge to stabilize it. The process requires expensive equipment to handle lime slurry. It frequently requires modification for removal of various metals to meet local environmental regulations.
It is known that a variety of compounds form complexes with metal ions, providing the potential to remove the metal ions from the compositions they contaminate. The capacity of humic and fulvic acids to complex metal ions is well established. For example, U.S. Pat. No. 6,143,692 (Sanjay et al.) discloses a process for removing metals from water using humic acid.
Kraft lignin and lignosulfonates are two classes of lignin derivatives available commercially. They are produced as by-products of the sulfate and sulfite pulping processes respectively. Lignosulfonates have been used in processes for the removal of various organic contaminants from water. Such processes are disclosed, for example, in U.S. Pat. No. 5,736,032 (Cox et al.), U.S. Pat. No. 4,933,087 (Markham, Jr. et al.) and U.S. Pat. No. 5,308,499 (Dixon et al.).
Lignosulfonates and kraft lignin contain an abundance of oxygen-containing functional groups (such as phenolic, hydroxyl, enolic, ether, carbonyl, and SO3H structures of various types), which are capable of forming lignin-metal complexes with high stability through ionic and covalent chemical bonding. It would be desirable to be able to use these plentiful and inexpensive lignin derivatives in processes for the effective removal of metal contaminants from acidic mine wastewater.